In a fuel cell stack which is a stack of polymer electrolyte fuel cells (PEFC), hydrogen gas supplied to an anode is ionized, conducts on a polymer electrolyte membrane to reach a cathode as proton, and reacts with oxygen supplied to the cathode and electrons to form water. Among the reaction enthalpies between hydrogen and oxygen, energy which is not used for generating power is converted into heat.
JP2002-208421A published by the Japanese Patent Office in 2002, discloses prevention of the moisture generated in the fuel cells from freezing by evacuating the fuel cells using dry gas or low humidity gas when the fuel cell stack stops operation at low temperature. This freeze prevention is particularly needed in a fuel cell stack for vehicles in which running conditions change sharply.